Phosphorescent materials

ABSTRACT

Phosphorescent materials and solutions suitable for their preparation, the solutions comprising a phosphorescence activator and a soluble condensation product capable of further reaction to form an insoluble condensation product, the latter being one that does not absorb ultraviolet radiation in the absorption range of the activator.

The present invention relates to materials or compositions for makingphosphorescent coatings and the application of such materials tosurfaces in order to render part or whole of the surfacesphosphorescent.

It is known that certain organic compounds which may or may not befluorescent when irradiated with short-wavelength light will exhibitphosphorescence during and after irradiation when dispersed in asuitable solid matrix, for example, in solid solutions or adsorbates.Such compounds are known as phosphorescence activators. It is known thatorganic phosphorescence (radiative decay of an excited triplet state)does not occur in the liquid phase, in which non-radiative decaypathways of the relatively long-lived triplet state are more favourable.Phosphorescent printing inks and coating compositions currentlyavailable therefore contain solid particles of phosphorescent materialdispersed in suitable liquid media.

British Patent No. 870,504 describes a phosphorescent material formed byreacting formaldehyde in liquid phase with a substance with which itwill react to give a solid condensation product, particularly an aminocompound such as urea or melamine, in the presence of a phosphorescenceactivator, to form a strongly phosphorescent solid condensation product.The phosphorescence activator is held in a solid matrix of formaldehyderesin. The material of British Pat. No. 870,504 is a brilliant phosphor;it has a high resistance to moisture and if deactivated by moistconditions can be restored to activity by drying. This material may beused as a phosphorescent ink after grinding to form a fine powder anddispersion in a liquid.

The present invention provides a composition consisting essentially of asolution of a phosphorescence activator and a soluble condensationproduct obtained by reaction of two or more components, which solublecondensation product is capable of further reaction to form an insolublecondensation product that does not substantially absorb ultravioletlight at any absorption frequency of the phosphorescence activator. Itis preferred that the insoluble condensation product is substantiallywithout absorption frequencies in the spectral range 250 to 380 nm.

The soluble condensation product, which will hereinafter be referred toas a prepolymer, is generally a material of low molecular weight. Theinsoluble condensation product, which has a high molecular weight, willbe referred to hereinafter as a resin. The formation of the resin fromthe prepolymer generally occurs by way of cross-linking.

The invention further provides a process for the preparation of asolution as described above, which includes partially condensing, insolution in the presence of a phosphorescence activator, two or morecomponents to form a prepolymer capable of further reaction to give asolid which does not substantially absorb ultraviolet light at anyabsorption frequency of the phosphorescence activator.

In a preferred embodiment, one of the components which are reacted toform the prepolymer, and, subsequently, the resin, is formaldehyde.Suitable substances with which formaldehyde will react to form a solidresin by way of a soluble prepolymer are amino compounds, especiallyurea and more especially melamine. As discussed in British Pat. No.870,504, melamine-formaldehyde resins are more resistant to moisturethan are urea-formaldehyde resin; this is advantageous in the presentcontext because of the sensitivity of phosphorescence to moisture.Resins formed from phenolic compounds and formaldehyde are unsuitablefor use with most phosphorescence activators because of ultravioletabsorption in the spectral range 250 to 380 nm.

The formation of an insoluble resin from formaldehyde and an aminocompound, for example melamine, occurs by way of two different chemicalreactions, which are favoured by different reaction conditions. Thefirst stage involves the reaction between the components to give amixture of monomeric and/or oligomeric methylolated melamines; theseproducts constitute the prepolymer. The formation of the prepolymer(methylolation) is favored by neutral or alkaline conditions. The secondstage is the cross-linking of the methylolated monomers and oligomers togive a high-molecular-weight resin; this is favored by acid conditions.

For the purposes of the present invention, the best results are obtainedif the methylolation reaction takes place substantially completelybefore cross-linking starts, since the functionality of theresin-forming monomers and oligomers (prepolymers) is determined by theextent of methylolation. The greater the functionality of themethylolated prepolymer, the higher the cross-link density, and hencethe moisture resistance, of the resin will be.

The solvent in which the activator and prepolymer are dissolved maysuitably be water or water mixed with an alcohol, preferably analiphatic alcohol having from 1 to 4 carbon atoms. The solution maycontain one or more stabilisers to inhibit premature conversion of theprepolymer to the finally desired cured resin.

The invention also provides a composition which includes a solution asdefined above in admixture or conjunction with one or more fillersand/or binders and/or pigments and/or stabilisers and/or otheradditives. These other components hereinafter termed adjuvants may ormay not be in solution in the solvent.

In one embodiment, the composition of the invention may be a coating mixfor coating the surface of paper to be printed; such paper may be used,for example, for the manufacture of postage stamps. In anotherembodiment, the composition of the invention may be a printing ink,which may be used, for example, for printing phosphorescent bars onsheets of postage stamps, and for applying address codes to mail for usein automatic sorting.

The present invention further provides a process for rendering part orwhole of a surface phosphorescent, which comprises the steps of:

(a) applying to the surface a composition which includes a solution, asdefined above, of a phosphorescence activator and a soluble condensationproduct capable of further reaction to give a solid condensationproduct, and

(b) forming the solid condensation product on the surface.

Step (b) may be carried out by various methods; for example, theevaporation of the solvent may be sufficient to cure the resin, or thesurface itself may have properties, for example, the pH, which enable itto catalyse the reaction. Curing may also be effected by exposure to anelevated temperature; if necessary, a curing agent may be added to thecomposition before application to the surface.

As activator, any substance capable of exhibiting phosphorescence whendispersed in a solid matrix may be used. Suitable activators are mainlyaromatic compounds, which may be benzenoid, heterocyclic or fused-ringcompounds. Aromatic amines, carbonyl compounds, carboxylic and sulphonicacids, ethers and phenols are particularly good activators; examples ofsuch compounds are listed in British Pat. No. 1,002,022.

Depending on the activator used, the phosphorescent material of theinvention may be excited at either long (365 nm) or short (254 nm)wavelength ultraviolet light. Examples are given:

    ______________________________________                                         Long-Wavelength (365 nm)                                                     Activators                                                                    Carbazole sulphonic acid                                                                          (blue afterglow)                                          Para-aminobenzophenone                                                                            (green afterglow)                                         α-naphthoflavone                                                                            (yellow afterglow)                                        8-amino-1-naphthol-3,6-disulphonic                                                                (orange-yellow afterglow)                                  acid                                                                         2-naphthylamine-6-sulphonic acid                                                                  (yellow-green afterglow)                                  Short-Wavelength (254 nm)                                                     Activators                                                                    Para-aminobenzoic acid                                                                            (violet afterglow)                                        Terephthalic acid   (blue-violet afterglow)                                   Fluorene sulphonic acid                                                                           (green-blue afterglow)                                    Diphenylene oxide sulphonic acid                                                                  (blue afterglow)                                          4,4'-dihydroxydiphenyl sulphonic acid                                                             (blue afterglow)                                          ______________________________________                                    

Phosphors containing short-wavelength activators are generallyunaffected by long-wavelength ultraviolet light, butlong-wavelength-excited phosphors do show a measurable response toshort-wavelength light. Despite this it is still possible to produce twoseparate sets of phosphorescent marks on one article and read themseparately. For example, if one code mark is first applied withshort-wavelength-excited phosphor, this can be detected by a suitablephotomultiplier for coding, and then a second set of marks applied withlong-wavelength-excited phosphor. These can be detected withoutinterference from the first set of marks.

For example, postage stamps may be printed with ashort-wavelength-excited phosphor, by means of which letters or otherarticles of mail may be segregated into first and second-class mail andthen "faced," i.e., stacked address upwards with the stamp in the topright-hand corner; address coding marks for automatic sorting may thenbe applied with a long-wavelength-excited phosphor, so that the stampphosphor does not interfere with the sorting process.

As previously mentioned, in one embodiment the invention relates to aprinting ink, and to printing phosphorescent marks on a surface,particularly on paper. An ink according to the invention may beproduced, for example, by reacting excess of a suitable amino compound,preferably melamine, with formaldehyde in aqueous solution, in thepresence of a phosphorescence activator, at a pH greater than about 6,preferably at a pH of about 9. The pH may be maintained at about 9, forexample, by means of a sodium carbonate/sodium borate buffer. The molarratio of melamine to formaldehyde is preferably within the range of from1.1 : 1 to 1.5 : 1. By careful control of the reaction conditions,especially solvent, temperature and pH, a low viscosity solutioncontaining about 20% by weight of a prepolymer of fairly low molecularweight may be obtained. The solution may be stabilised by the additionof an alcohol, preferably methyl, ethyl, isopropyl or n-butyl alcohol,preferably in an amount of up to 20% by volume, advantageously 10 to 15%by volume. Inks differing slightly in properties such as surface tensionand viscosity may be obtained by variation of the formulation. Inksaccording to the invention may have a shelf life of at least 6 monthsunder normal storage conditions without curing of the prepolymer takingplace.

A printing ink according to the invention may be completelynon-particulate, and is therefore very suitable for use in jet printers;conventional particulate phosphor inks tend to block the nozzles of jetprinters by agglomeration of the particles. Forced drying of the printedsurface may be used to accelerate evaporation of the solvent and curingof the resin; for example, an infra-red beam parallel to the surface andslightly above it and focused above the actual printing area may beused, so that the drops of ink are almost dry by the time they hit thesurface. The use of a beam parallel to the surface ensures that thesurface itself is not damaged; such a system may be achieved usingappropriate reflecting and focusing devices. Use of paper having aweakly acid reaction also accelerates curing.

In another embodiment, the invention is concerned with an overallcoating for paper which can be rendered phosphorescent by exposure toultraviolet radiation. The invention accordingly provides apaper-coating composition comprising a solution, preferably an aqueoussolution, according to the invention, in admixture or conjunction withone or more pigments and/or binders and/or other componentsconventionally used in a paper-coating composition. The coatingcomposition according to the invention may be prepared by mixing theprepolymer solution of the invention with a conventional coating mix,which generally consists of an aqueous suspension of pigments, bindersand other additives. The wet-coating composition should preferably notcontain more than 20% by weight of the prepolymer solution, otherwisedifficulties may be experienced when printing on the coated papers.

If desired, a curing agent (cross-linking agent) may be added to theprepolymer solution immediately before mixing with the coating mix.

The coating composition may be applied to paper using conventionalequipment; the coated paper may then be dried, for example by passing itthrough a heated tunnel or over heated rollers. If the degree of heatingis insufficient to cure the resin completely at this stage, the curingwill continue slowly at ambient temperature.

The preparation of an aqueous solution according to the invention foruse in a paper coating mix is preferably carried out at a pH within therange of 6 to 9, preferably 6 to 8, and more preferably 6 to 7,preferably at a temperature of from 70° to 80° C., for example, 75° C.The higher the pH of the reaction mix, the longer the reaction takes atany given temperature and the more closely the degree of reaction of thefinal product may be controlled. The pH may be adjusted by means of, forexample, sodium hydroxide or calcium carbonate. In the case of calciumcarbonate the pH may be maintained at approximately 6.2 by the presenceof excess of this reagent. The extent of reaction in the formation ofthe soluble prepolymer is important and may conveniently be monitored bymeasurement of the miscibility of the reaction mixture with water, i.e.,titration with water until the mixture becomes turbid due toprecipitation of a solid resin; the maximum volume of water with whichone volume of prepolymer solution may be mixed without causing turbidityis commonly referred to as the water tolerance of the solution. Theextent of reaction may alternatively be monitored by measurement of theviscosity of the reaction mixture. This method must be used if calciumcarbonate is used for adjustment of pH, since excess of this reagentrenders the solution turbid so that water tolerance is difficult todetermine.

In a preferred embodiment, the conversion of the soluble condensationproduct (prepolymer) to the solid resin is promoted by the addition of acatalyst, to promote cross-linking of the prepolymer, immediately beforethe prepolymer solution is added to the other components of the coatingmix; the catalyst is preferably an acid, for example sulphuric or aceticacid.

The extent of reaction in the formation of the soluble condensationproduct (prepolymer) is important if it is to be used in a coating mixsince if it is not sufficiently reacted at this stage the resistance ofthe final coating to de-activation of phosphorescence by moisture isimpaired, while if it is reacted too far the prepolymer solution willnot form a homogeneous mixture with the coating formulation and theproduct then has unsatisfactory coating and printing properties. Thusthe most satisfactory results are obtained when the reaction is allowedto go as far as possible as is consistent with the production of ahomogeneous coating mix. The extent of reaction allowable depends on thenature of the pigment/binder system used and also on the solids contentof the mix.

For the preparation of a typical coating composition having a 50% solidscontent and containing 15-20% of the prepolymer solution, a solution ofwater tolerance of 2.0 to 2.5 volumes of water to 1 volume prepolymersolution is suitable. The viscosity which corresponds to this watertolerance is dependent on the pH of the prepolymer solution. Examples ofapproximate viscosity values, together with reaction times, at differentpH values, for completion of the prepolymer stage, are given below:

    ______________________________________                                                                      Viscosity of                                                                  prepolymer solution                                              Reaction Time                                                                              at water tolerance                              pH   Temperature (minutes)    (cp)                                            ______________________________________                                        8.5  75° C                                                                              190          13-15                                           7.4  "           60           10-12                                           6.0  "           25           7-9                                             9.0  92° C                                                                              70           12-14                                           ______________________________________                                    

Paper coated with a coating composition according to the invention maybe used, for example, in the production of postage stamps; for thisapplication, a short-wavelength-excited activator, for example,p-amino-benzoic acid (PAB) or terephthalic acid (TPA), is preferablyused, to prevent interference with automatic sorting processes in whichlong-wavelength-excited address code markings are used.

The following Examples illustrate the invention: in Example 3 referenceis made to FIGS. 1 and 2 of the accompanying drawings.

EXAMPLE 1 Preparation of a printing ink

850 g melamine (1.3 moles) were mixed with 400 g 37% aqueousformaldehyde (1 mole), the suspension was stirred, and 300 ml buffersolution (sodium carbonate/sodium borate pH 9.2) added to give a pH ofabout 9. 20 g carbazole-sulphonic acid sodium salt were then added(approximately 14% by weight based on formaldehyde) and sufficient waterwas added to give a total liquid volume of about 21/2 liters. Thesuspension was stirred and heated to reflux, and this temperature wasmaintained for 50-60 minutes. The heat source was removed and 300-400 mlmethanol were added. Stirring was continued until the mixture hadcooled. It was then allowed to stand at room temperature for about 18hours. The liquid was then decanted; this yielded about 21/2 liters ofink.

The recovered solid (350 g) was washed with 50/50 alcohol/water, driedand re-used as follows:

350 g recovered solid was mixed with 450 g of fresh melamine; 300 ml ofbuffer pH 9.2 and 350 g of 37% aqueous formaldehyde solution were thenadded. After addition of the carbazolesulphonic acid sodium salt andwater, the preparation was carried out as described above. A furtheryield of about 21/2 liters of ink was obtained.

The aqueous ink had a low viscosity and contained about 20% by weight ofa fairly low molecular weight prepolymer. About 10 to 15% by volume ofalcohol was added as stabiliser and the solution was stored for 6months; no obvious increase in viscosity had occurred at the end of thisperiod.

The ink was applied to paper in a thin film; it became smudge-free inabout a second and became increasingly phosphorescent as it dried. Whenthe film was wetted, the phosphorescence was quenched, but after dryingthe film was again phosphorescent.

EXAMPLE 2 Preparation of a paper-coating mix

24 g p-aminobenzoic acid and 6.5 g sodium hydroxide were dissolved withwarming in 75 ml water; the pH of this solution was about 9. Thesolution was added to 1.8 liters of 40% aqueous formaldehyde solution,the mixture was heated to 75° C. and the pH adjusted to 8 by theaddition of further alkali. 1.2 kg melamine powder was added and thetemperature of the mix was maintained at 75° C. with efficient stirring.The melamine dissolved to give a clear yellow solution after 10-15minutes. The pH was maintained at 8.

Half an hour after the addition of the melamine, a 10 ml sample waswithdrawn from the solution and titrated with water until the mixturebecame turbid. Samples were taken at 10 to 15 minutes intervals. As thereaction proceeded, the quantity of water required to precipitate solidresin from the solution decreased and the viscosity of the prepolymersolution increased. When the reaction had proceeded to a point at which21/2 - 3 volumes of water were sufficient to produce turbidity in 1volume of solution, the solution was mixed with an aqueous coating mixconsisting of pigment and binder, as described previously, with constantstirring. The resulting wet coating composition contained about 20percent by weight of prepolymer solution.

EXAMPLE 3

A series of experiments were carried out, in which solutions wereprepared according to the procedure of Example 2 under various pH andtemperature conditions:

(a) Temperature 75° C., pH of mixture 6.1

(b) Temperature 75° C., pH of mixture 7.4

(c) Temperature 75° C., pH of mixture 8.5

(d) Temperature 92° C., pH of mixture 9.0

The amount of water required to make the reaction mixture turbid wasmeasured as a function of reaction time; the results are shown in FIG. 1of the accompanying drawings, wherein t is the reaction time in hoursand V is the number of volumes of water required to product turbidity in1 volume of reaction solution. It was found that a prepolymer solutionwhose extent of reaction is within the area above the line A produces asmooth homogeneous coating composition when mixed with a conventionalcoating mix containing pigment and binder in suitable quantities.Solutions whose extent of reaction is in the area below the line B willnot give satisfactory coating mixes. Between the lines A and B, ratherthick but otherwise satisfactory mixes are obtained.

It may be seen that, out of the four sets of reaction condition tried, apH of 8.5 and a temperature of 75° C., gave the slowest, and thereforemost controllable, reaction.

FIG. 2 shows the viscosity of the reaction mixture, η, in cp, as afunction of reaction time t in hours. In addition to the four runs (a)to (d), a fifth run (e) was carried out at pH 9.7 and 75° C. The shadedportion of each curve indicates where the resin solution produced athick or unsatisfactory coating mix.

EXAMPLE 4 Preparation of a coating mix -- alternative procedure

24 g p-aminobenzoic acid was dissolved in 200 ml of hot water, andprecipitated calcium carbonate solution was slowly added untileffervescence ceased; about 25 g calcium carbonate was required. Thesolution had a pH of about 6.2.

The activator solution was added to 1.8 liters 40% aqueous formaldehydesolution, and the pH was maintained at 6.2 by the addition of morecalcium carbonate if necessary.

Melamine (1.2 kg) was added and the reaction carried out at 75° C. as inExamples 2 and 3. At this pH, however, the reaction proceeds morerapidly than at the higher pH used in the previous Examples; it wascomplete in 20-30 minutes.

The extent of reaction was monitored by measurement of the viscosity ofthe solution. The end point was taken to be when the solution had aviscosity of 10- 12 cp. A coating mix was then prepared as in Example 2.

EXAMPLE 5 Preparation of a coating mix containing an acidic curing agent

An activator solution was prepared by dissolving 30 g p-aminobenzoicacid and 8 g solid sodium hydroxide in 250 ml water with heating; thesolution had a pH of about 7. 130 ml of this solution were added to 1200ml formaldehyde and the pH was adjusted to about 6.5.

The temperature of the mixture was raised to 75° C. and 1200 g melaminepowder were added with constant stirring. Reaction of the melamineformaldehyde mix to the prepolymer stage was continued with stirring at75° C. until the water tolerance of the solution, determined asdescribed above, was about 21/2 : 1. The time required for the reactionwas approximately 40 mins. Sulphuric acid (24 ml of a solution preparedby mixing 100 ml concentrated sulphuric acid and 750 ml water) was addedto the solution with vigorous stirring and immediately afterwards theresulting solution was added to approximately 15 kg of coating mix, thesolution/coating mix proportion having been chosen to give the requiredphosphorescent intensity.

We claim:
 1. A composition consisting essentially of an aqueoussolution, having a pH within the range of from 6 to 9.5 of aphosphorescence activator and a soluble precondensate obtained by areaction between formaldehyde and an amino compound, the solubleprecondensate being capable of further reaction in the absence of thephosphorescence activator whereby there is formable an insolublecondensation product that does not substantially absorb ultravioletradiation at any wavelength at which the phosphorescence activatorsubstantially absorbs ultraviolet radiation, and being capable of suchfurther reaction in the presence of the phosphorescence activatorwhereby there is formed a phosphorescent insoluble condensation product.2. A composition as claimed in claim 1, wherein the insolublecondensation product formable by further reaction in the absence of thephosphorescence activator does not substantially absorb ultravioletradiation at any wavelength within the range of from 250 to 380 nm.
 3. Acomposition as claimed in claim 1, wherein the soluble precondensate hasbeen obtained by the reaction of the amino compound, in a molar excess,with formaldehyde.
 4. A composition as claimed in claim 3, wherein thesoluble precondensate is the product of the reaction of from about 1.1to about 1.5 moles of melamine per mole of formaldehyde.
 5. Acomposition as claimed in claim 1, wherein the phosphorescence activatoris selected from the group consisting of aromatic amines, aromaticcarbonyl compounds, aromatic carboxylic acids and their salts, aromaticsulphonic acids and their salts, aromatic ethers, and aromatic hydroxycompounds.
 6. A composition as claimed in claim 1, wherein thephosphorescence activator is selected from carbazole sulphonic acid andsalts thereof, and p-aminobenzoic acid and salts thereof.
 7. Acomposition as claimed in claim 1, also containing at least one adjuvantselected from the group consisting of fillers, binders, pigments andstabilizers.
 8. A process for the preparation of a composition asclaimed in claim 1, which includes partially condensing formaldehyde andamino compound components, in aqueous solution at a pH within the rangeof from 6 to 9.5 and in the presence of a phosphorescence activator, toform a soluble precondensate capable of further reaction to give aphosphorescent insoluble condensation product that does notsubstantially absorb ultraviolet radiation at any wavelength at whichthe phosphorescence activator substantially absorbs ultravioletradiation.
 9. A process as claimed in claim 8, wherein of saidcomponents formaldehyde and an amino compound, the amino component is ina molar excess.
 10. A process as claimed in claim 9, wherein thecomponents comprise formaldehyde and melamine and are reacted in a molarratio within the range of from about 1:1.1 to about 1:1.5.
 11. A processas claimed in claim 8, wherein the partial condensation is carried outat a pH within the range of from 8.5 to 9.5.
 12. A process as claimed inclaim 8, wherein at least one aliphatic alcohol is added to the aqueoussolution after the partial condensation.
 13. A process as claimed inclaim 12, wherein the alcohol is added until the aqueous solutioncontains from 10 to 15 volume percent of alcohol.
 14. A process asclaimed in claim 8, wherein the phosphorescence activator is selectedfrom the group consisting of aromatic amines, aromatic carbonylcompounds, aromatic carboxylic acids and their salts, aromatic sulphonicacids and their salts, aromatic ethers, and aromatic hydroxyl compounds.15. A process as claimed in claim 8, wherein the phosphorescenceactivator is selected from carbazole sulphonic acid and salts thereofand p-aminobenzoic acid and salts thereof.
 16. A printing inkcomposition consisting essentially of an aqueous solution of aphosphorescence activator and a soluble precondensate obtained byreacting formaldehyde with a molar excess of an amino compound, whichsoluble precondensate is capable of further reaction in the presence ofthe phosphorescence activator whereby there is formable a phosphorescentinsoluble condensation product that does not substantially absorbultraviolet radiation at any wavelength at which the phosphorescenceactivator absorbs ultraviolet radiation.
 17. A printing ink compositionas claimed in claim 16, wherein the soluble precondensate has beenobtained by reacting formaldehyde with a molar excess of melamine.
 18. Aprinting ink composition as claimed in claim 17, wherein the solubleprecondensate is the product of the reaction of from about 1.1 to about1.5 moles of melamine per mole of formaldehyde.
 19. A printing inkcomposition as claimed in claim 16, having a pH within the range of from8.5 to 9.5
 20. A printing ink composition as claimed in claim 19,containing sodium carbonate and sodium borate as a buffer.
 21. Aprinting ink composition as claimed in claim 16, containing at least onestabilizer.
 22. A printing ink composition as claimed in claim 21,wherein the stabilizer is an aliphatic alcohol.
 23. A printing inkcomposition as claimed in claim 22, wherein the aqueous solutioncontains from about 10 to about 15 percent by volume of alcohol.
 24. Aprinting ink composition as claimed in claim 16, wherein thephosphorescence activator is selected from the group consisting ofaromatic amines, aromatic carbonyl compounds, aromatic carboxylic acidsand their salts, aromatic sulphonic acids and their salts, aromaticethers, and aromatic hydroxy carbazole compounds.
 25. A printing inkcomposition as claimed in claim 16, wherein the phosphorescenceactivator is selected from carbazole sulphonic acid and salts thereof,and p-aminobenzoic acid and salts thereof.
 26. A printing inkcomposition consisting essentially of an aqueous solution, having a pHwithin the range of from 8.5 to 9.5, of a phosphorescence activator anda soluble melamine-formaldehyde precondensate, and including astabilizing amount of an aliphatic alcohol having from 1 to 4 carbonatoms.
 27. A printing ink composition consisting essentially of anaqueous solution of a phosphorescence activator and a solublemelamine-formaldehyde precondensate in the form of a mixture of speciesselected from monomeric and oligomeric methyloated melamines.
 28. Apaper-coating composition which comprises (a) an aqueous coating mix ofpigment and binder and (b) an aqueous solution, having a pH within therange of from 6 to 8, of a phosphorescence activator and a solubleprecondensate of formaldehyde and an amino compound, which solubleprecondensate is capable of further reaction in the presence of thephosphorescence activator whereby there is formable a phosphorescentinsoluble condensation product that does not substantially absorbultraviolet radiation at any wavelength at which the phosphorescenceactivator substantially absorbs ultraviolet radiation.